Saposin B Deficiency With Neurologic and Hepatobiliary Involvement: Two Patients Expanding the Clinical Spectrum

IntroductionSaposin B deficiency is an ultrarare lysosomal disorder caused by biallelic mutations in the <i>PSAP</i> gene. Although it clinically resembles classical arylsulfatase A (ARSA)-deficient metachromatic leukodystrophy (MLD), ARSA activity remains biochemically normal. Fewer than 30 genetically confirmed patients have been reported to date.Patient PresentationWe describe 2 pediatric patients with homozygous <i>PSAP</i> gene mutations. Both presented with progressive neurodegeneration, spastic quadriparesis, demyelinating peripheral neuropathy, and radiologic findings consistent with MLD. The first patient harbored a canonical splice-site variant (c.577-1G>T), whereas the second carried a recurrent missense mutation (p.Cys241Ser). Notably, the first patient also had a <i>PMP22</i> duplication consistent with coexisting CMT1A. Despite preserved ARSA activity, both patients fulfilled clinical and imaging criteria for MLD. Abdominal imaging revealed gallbladder sludge in both patients, with additional biliary tract dilation in one. These hepatobiliary findings have not been previously reported in genetically confirmed saposin B deficiency, broadening the known phenotypic spectrum.ConclusionThese patients underscore the importance of <i>PSAP</i> gene sequencing in MLD-like presentations with normal ARSA activity and suggest that hepatobiliary involvement may be an underrecognized feature of saposin B-related MLD.